Basketball, tennis, running, and aerobics are but a few of the many popular athletic activities which produce a substantial impact on the foot when the foot strikes the ground. To cushion the strike force on the foot, as well as the leg and connecting tendons, the sole of shoes designed for such activities typically include several layers, including a resilient, shock absorbent layer such as a midsole and a ground contacting outer sole or outsole which provides both durability and traction.
The typical midsole uses one or more materials or components which affect the force of impact in two important ways, i.e., through shock absorption and energy absorption. Shock absorption involves the attenuation of harmful impact forces to thereby provide enhanced foot protection. Energy absorption is the dissemination of both impact and useful propulsive forces. Thus, a midsole with high energy absorbing characteristics generally has a relatively low resiliency and, conversely, a midsole with low energy absorbing characteristics generally has a relatively high resiliency. The optimum midsole should be designed with an impact response that takes into consideration both adequate shock absorption and sufficient resiliency.
One type of sole structure in which attempts have been made to design appropriate impact response is soles, or inserts for soles, that contain a bladder element of either a liquid or gaseous fluid. These bladder elements are either encapsulated in place during the foam midsole formation or dropped into a shallow, straight walled cavity of a foam midsole and cemented in place, usually with a separate piece of foam cemented on top. Gas filled bladder elements are disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy, the contents of which are hereby incorporated by reference. A bladder or barrier member is formed of an elastomer material having a multiplicity of preferably intercommunicating, chambers which are inflated to a relatively high pressure by a gas having a low diffusion rate through the bladder. The gas is supplemented by ambient air diffusing through the bladder to thereby increase the pressure therein and obtain a pressure that remains at or above its initial value over a period of years. (U.S. Pat. Nos. 4,340,626, 4,936,029 and 5,042,176 to Marion F. Rudy describe various improvements in such bladders and are also hereby incorporated by reference.) The bladder insert is incorporated into the shoe, in the '156 patent, by placement within a cavity below the upper, e.g., on top of a midsole layer and within sides of the upper or within the midsole. In the '945 patent, the bladder insert is encapsulated within a yieldable foam material, which functions as a bridging moderator filling in the irregularities of the bladder with foam pillars, providing a substantially smooth and contoured surface for supporting the foot and forming an easily handled structure for attachment to an upper. However, maintaining quality and density consistency in the pillars of foam is a difficult manufacturing task. In addition, the presence of the moderating foam detracts from the shock absorbing function of the gas inflated bladder, as well as the cushioning perception provided by the bladder. Thus, when the inflated bladder is encapsulated in a foam midsole, the favorable cushioning response characteristics of the inflated bladder are reduced by the presence of the encapsulating foam, including the pillars that fill in the irregularities of the bladder.